Two isomers of 1,2-dichloroethene are known. One has a dipole moment of 2.4 D; the other has zero dipole moment. Draw the two isomers, and explain why one has zero dipole moment.
CHCl=CHCl 1,2-dichloroethene

Verified step by step guidance

1. Identify the two geometric isomers of 1,2-dichloroethene: cis-1,2-dichloroethene and trans-1,2-dichloroethene.
2. Draw the structure of cis-1,2-dichloroethene, where both chlorine atoms are on the same side of the double bond. This arrangement leads to a net dipole moment because the dipoles from the C-Cl bonds do not cancel each other out.
3. Draw the structure of trans-1,2-dichloroethene, where the chlorine atoms are on opposite sides of the double bond. In this configuration, the dipoles from the C-Cl bonds are equal in magnitude but opposite in direction, leading to cancellation and a net dipole moment of zero.
4. Explain that the dipole moment is a vector quantity, and in the trans isomer, the symmetry of the molecule causes the dipole vectors to cancel each other out, resulting in no net dipole moment.
5. Conclude that the cis isomer has a dipole moment of 2.4 D due to the additive effect of the dipole vectors, while the trans isomer has a dipole moment of zero due to the cancellation of the dipole vectors.

Key Concepts

Isomerism

Isomerism refers to the phenomenon where compounds have the same molecular formula but different structural arrangements. In the case of 1,2-dichloroethene, the two isomers are geometric isomers, specifically cis and trans isomers, which differ in the spatial arrangement of the chlorine atoms around the double bond.

Dipole Moment

A dipole moment is a measure of the separation of positive and negative charges in a molecule. It is a vector quantity, with both magnitude and direction. Molecules with symmetrical charge distribution, like the trans isomer of 1,2-dichloroethene, can have a net dipole moment of zero, while asymmetrical ones, like the cis isomer, have a non-zero dipole moment.

Cis-Trans Isomerism

Cis-trans isomerism is a type of stereoisomerism where the relative orientation of functional groups within a molecule differs. In 1,2-dichloroethene, the cis isomer has both chlorine atoms on the same side of the double bond, resulting in a net dipole moment, while the trans isomer has them on opposite sides, canceling out the dipole moment.